1. Field of the Invention
The present invention pertains to a technology of automatically determining collection conditions for complete-solution cone beam helical reconstruction, and especially to an X-ray CT device for radiating a living body with X-rays and thereby acquiring internal body information as an image.
2. Description of the Related Art
In the field of X-ray CT devices, a system of collecting projection data from various angles with an X-ray tube for producing an X-ray beam and an X-ray detector positioned across a subject from the X-ray tube rotating around the subject is defined as the third-generation CT. The conventional technologies adopt an X-ray beam of a fan shape and a detector of a one-dimensional array type.
Scanning systems can be divided into two types, conventional scanning and helical scanning. In the conventional scanning, the X-ray tube moves around on the same track. On the other hand, the helical scanning is defined as the X-ray source and the detector continuously rotating around a subject while a bed carrying the subject moves along the direction of the body axis in synchronization with the rotation of the X-ray source and the detector. The helical scanning has been given this name because the X-ray tube moves on a helical track when the coordinates of the tube moving along with the subject are considered. The distance between the positions of the X-ray tube that change in the body-axis direction, or in other words the z-axis direction, in one rotation is defined as a helical pitch.
In addition, a CT incorporating a detector of a two-dimensional array type has been known as a third- or fourth-generation CT. This CT is provided with an X-ray tube that generates an X-ray beam of a cone shape spreading in the body axis-direction, unlike an X-ray of a fan shape, and an X-ray detector that is formed by putting, for example, N rows of one-dimensional array detectors together in the z-axis direction so as to arrange detector elements in a matrix. Such a device is called a cone beam CT.
The complete-solution cone beam helical reconstruction proposed by A. Katsevich in University of Central Florida allows for an image quality level without a cone beam artifact. The method of reconstruction is suggested by Jpn. Pat. Appln. KOKAI Publication No. 2006-095297, for example.
The complete-solution helical reconstruction developed by Katsevich having different bed moving speeds, 1PI and 3PI, allows for an image quality level without cone beam artifacts. The speeds 1PI and 3PI represent different bed moving speeds with respect to the width of the detector for detecting X-rays. 1PI is a bed moving speed at which data equivalent to 180 degrees becomes available, while 3PI is a bed moving speed lower than 1PI, at which data equivalent to 540 degrees becomes available.
In general, the user determines the optimal bed moving speed from a target scanning range and the maximum length of a patient's breath holding time. However, in Katsevich's reconstruction system, the bed moving speed is limited only to 1PI and 3PI, and thus the user is not allowed to freely select a bed moving speed. For instance, in the 0.5-mm×64-row data collection on an X-ray detector of a two-dimensional array type, the bed moving speed can be selected only from 17.8 mm/rotation (3PI) and 45.7 mm/rotation (1PI). The reconstruction could be performed at other bed moving speeds, but the data use efficiency is lowered, only unnecessarily exposing the patient to radiation.